Eat a pie that is well known in Thessaloniki and it´s called 'bougatsa' and it is with cheese or with cream.
Eat greek kebab to one of the millions fast-food places and taste the crepes in Navarinou square.
Walk by the sea and enjoy your coffee to one of the numerous cafe by the sea, or you can enjoy in the old town from where you can have a wonderful view of the city with the sea.
Iskembe joints - Iskembe is a tripe based soup that claims to prevent hangovers as long as you eat it at 3:00am. In any large town or city there will be somebody selling Iskembe when you need it.
Fondest memory: what you see in the photo is not iskembe . iskembe is unphotographable .......
Seafood is excellent almost everywhere but try an 'ouzery' called Melathron, get a relationship with a waiter (there's an italian soccer team of Juventus fan which looks like Zidane :-) and let him to suggest you best dishes (mezedes). They actually have this habit: they do not order to a waiter, they really speak with him for few minutes in order to understand which is the best daily dish ... the only problem is that you should learn a little bit of greek !
Fondest memory: Ice coffees !!!